It's silly writing.
To me, if you want to make Carl a real obstacle for Wemma, actually give Carl and Emma a magical, romantic scene together. Marrying him doesn't convince me that she might, at the end of the day, TRULY prefer being with Carl.
And, in order to think there's a chance Wemma won't happen, you need to convince me of that.
And they did nothing of the sort. They showed they don't give a crap about Carl and Emma. Everything was shown off screen except for the one scene they dubbed important which was Emma's look of regret.